It has been proposed previously to provide various types of crash sensor in a motor vehicle. Typically the crash sensors generate output signals which are used to control the deployment of safety devices such as air-bags, or pretensioners which pretension safety-belts.
It is known that a crash sensor located close to the outer skin of the vehicle detector crash more quickly than a central accelerometer which is secured to the chassis of the vehicle.
Many vehicles are provided with sensors located in the doors or other parts of the side of the vehicle and also in the region of the front and rear bumpers or fenders of the vehicle, in addition to a central accelerometer.
If the sensors located close to the outer skin of the vehicle are accelerometers, such accelerometers will be exposed to a high acceleration when that part of the outer skin is deformed in a crash. However, the accelerometer signal will represent the acceleration of the accelerometer housing, which is located only at one point on the skin of the vehicle, and the acceleration at one point may be very different to the acceleration at an adjacent point, especially if there is an impact with a relatively small object such as, for example, a street light or lamp-post.
In a typical situation, an impact will cause the outer skin of the vehicle to change its speed, so that, over the course of at most a few milliseconds, the outer skin changes its absolute speed from the speed of the vehicle to the speed of the impacting object, because the outer skin of the vehicle is relatively soft. An accelerometer which is mounted adjacent the skin of a vehicle will thus sense one very high pulse of acceleration in a typical impact situation.
A centrally mounted accelerometer which is mounted on the chassis of the vehicle, for example, will sense acceleration, in an impact situation, much later than an accelerometer located adjacent the outer skin of the vehicle in the region of the impact. The reason for this is that, in many impact situations, the chassis of the vehicle will only accelerate or decelerate substantially when a “crumple zone” has crumpled, absorbing some of the energy of the impact.
Typically, a safety device such as an air-bag or seat-belt pretensioner is triggered in response to the integrated acceleration signal from one or more accelerometers exceeding a predetermined threshold level. This means that if the impact speed is low, no triggering of the safety device will occur.
To provide an optimum degree of response, a vehicle should be provided with a central accelerometer and also accelerometers located adjacent the outer skin of the vehicle in all regions of the vehicle where an impact could occur. If the sensors located adjacent the outer skin of the vehicle are accelerometers, a substantial number of accelerometers will be needed, and this is expensive.
The present invention seeks to provide an improved crash sensor arrangement.